Plotting Atlantic and Eastern Pacific Tropical Cyclone Paths in IDV: National Hurricane
Abstract: In this
exercise, which is quite elementary, you will learn how to obtain the
latest expected trajectory information from the NHC for any of the
identified storms in the Atlantic (or Pacific if you take that initial
option). The data can downloaded as a zipped set of three shapes:
A point shape, indicating the NHC's best guess about future locations of
the storm at stated times; a line shape indicating the NHC's best guess
about the storm's future path; and a polygon shape showing the NHC's best
guess about the possible spread of storm locations (given the natural
uncertainties) in days 1-3, and in days 4-5 from the present. THIS LESSON IS PROVIDED FOR
EDUCATIONAL PURPOSES ONLY. NO ONE SHOULD ATTEMPT TO RELY ON THESE
ANALYSIS PRODUCTS FOR NAVIGATION OR FOR ANY OPERATIONAL PURPOSES AT SEA.
Preliminary Reading (in
OceanTeacher, unless otherwise indicated):
|1. Open the NHC website and
read the general information about its activities and products. Here
you see the EASTERN PACIFIC version of the homepage.
|2. Here, we've clicked over
to the ATLANTIC home page.
|3. Click on any available
hurricane icon to see all these detailed products. Hurricane Katia was
present when this exercise was written.
If there is no current tropical
cyclone, then this exercise isn't really useful.
|4. Click on
DOWNLOAD GIS DATA and save the file to the folder DATA > OCEAN with the name
al122011_5day_latest_on_20110906T1518Z-6.zip which is a concatenation
of the filename offered by NHC (al122011_5day_latest) and additional
information to specify the exact time the file was obtained. Note that
Z-6 is the ISO specification for the Eastern Time Zone in the USA.
|5. Unzip the file
in place. You'll find these shapes:
- al122011.034A_5day_pts.shp - Point shape, probably of sequential
storm center locations
- al122011.034A_5day_pgn.shp - Polygon shape, probably of storm location
- al122011.034A_5day_lin.shp - Line shape, probably of expected storm
- al122011.034A_ww_wwlin.shp - Unknown object that doesn't plot in a
standard GIS (Saga)
|6. In a "standard GIS"
program, the first three of the above files plot as you see here. The
points are located 12 hours apart (for the first 5 locations) and 24 hours
apart (for the final 3 locations).
The IDV documentation warns that point
shapes don't work in IDV, without additional work, because it cannot handle
one-dimensional points. You can use the Saga tool MODULES >
SHAPES/TOOLS > BUFFER SHAPE to create the small circles you see here.
We won't bother to do that during this lesson, but we'll confirm the point
shape incompatibility issue in the panels below.
|7. Run IDV.
|8. Use DATA > DATA CHOOSER
and navigate to the OCEAN data files. Select DATA SOURCE TYPE:
SHAPEFILE. Then select the line, polygon and points SHP files.
Then click ADD SOURCE.
|9. In the FIELD SELECTOR,
select the line shape object, and click CREATE DISPLAY.
|10. After clicking past a
display settings window, you'll see this trajectory path for the storm for
the next 5 days.
|11. In the same way, select
the polygon shape, and click CREATE DISPLAY.
|12. Here are the combined
shapes. If you use a figure like this, be very careful to tell your
readers/users that these are estimated locations, based on models, but the
actual storm path could vear dramatically outside these cones.
|13. You can also
try to visualize the point shape, in the same way, but nothing will appear.
|14. Now, you can clean up
your graphic, by selecting the line shape in the right-hand list of objects.
Then select DISPLAYS > EDIT > PROPERTIES
|15. Edit the DISPLAY LABEL,
as you see here. Then click OK.
You should, in the same way, select
the point shape object and delete its DISPLAY LABEL. Then click OK.
Also select the polygon shape object and delete its DISPLAY LABEL.
Then click OK.
|16. Here you have a nice
graphic with only one simple label, showing the probably path and the
|17. A FINAL WORD
OF CAUTION. If you are not a certified meteorologist, then do not use
the above methods and products to "publish" or circulate tropical storm
analyses as if they were your own invention. Make certain that
everything you make here is clearly identified by you as having come from a
specific other source, namely the National Hurricane Center of the USA.
And be very careful that you date your files and products so that older
version can be scrapped and do not cause problems. These are wonderful
data products and software systems, but they can easily be abused and cause
difficulties if you do not take care to educate users about their origins